Unit Name: Gravelbourg Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Bajocian (171.6 - 167.7 ma)
Province/Territory: Saskatchewan; Minnesota
Originator: Milner, R.L. and Thomas, G.E., 1954.
Type section is in the Tidewater Eastend Crown No. 1 well in Lsd 15, Sec. 11, Twp. 6, Rge. 20W3M, southeastern Saskatchewan, between 1,469 and 1,497 m (4,820 and 4,910 ft).
Some 27 m (89 ft) thick in the type area, it thins westward into Alberta onto the flank of the Sweetgrass Arch. Towards the Williston basin centre it thickens to over 60 m (197 ft). It thins to 18 m (59 ft) toward the basin margin in southwestern Manitoba, where it changes character by the presence of an erosional unit of fine- to medium-grained sandstone that truncates the lower member.
Divided into lower and upper members. The lower member comprises buff to tan dolomitic limestone, both dense and laminated with partings of green shale. Its upper part features algal forms, oolite, pelecypod shell fragments and glauconite. Veins and bands of bluish white chalcedonic chert replacing limestone and vugs of anhydrite occur over its lower half. Limestone overlies a basal 2 m (7 ft) of dark green and brown shale with abundant fish scales and inclusions of anhydrite. The upper member consists mostly of dark greenish grey fossiliferous shale which, in southeastern Saskatchewan is interbedded with minor amounts of quartzose sand and argillaceous limestone. The upper contact is marked by thin interbeds of solution pitted, tan calcilutite transitional to the Shaunavon Formation.
Overlies the Watrous Formation, but overlaps it onto the Mississippian Madison Limestone in southeastern Alberta. The lower member is equivalent to the Reston Formation of Manitoba and the Kline Member of the Nesson Formation in North Dakota and Montana. The upper member is equivalent to the lower part of the Melita Formation of Manitoba and the Tampico Shale of the Piper Formation in North Dakota and Montana.
Milner and Thomas, 1954; Nordquist, 1955; Peterson, 1972; Stott, 1955.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: J.E. Christopher
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 19 May 2004